You probably have a Birmingham made gun that used Belgian made barrel tubes . Belgian "damascus" was imported into the UK up untill 1939 . It was cheaper than British made and also the Belgians were producing it long after the British stopped . The way it was made also gave some very fancy patterns that could make a low end gun look more attractive.
I can only say I have never seen an Army and Navy gun that was not of English manufacture and as it bears English proof marks the gun was a least finished in England even if the tubes were sourced from Belgium .
